1. Field of the Invention
This invention relates to a rotation control device for a magnetic disc drive apparatus.
2. Discussion of the Prior Art
A magnetic disc drive apparatus includes a magnetic disc rotated by a drive means and a magnetic head floating a short distance from the magnetic disc, whereby information is written on the magnetic disc and read from the magnetic disc by the magnetic head.
Recently used has been the currently so-called CSS (Contact Start Stop) type for the magnetic head of the magnetic disc drive apparatus wherein the magnetic head contacts the magnetic disc when it is not rotated, and wherein the magnetic head begins to float from the magnetic disc when the rotation speed of the disc reaches a particular level. In this type of magnetic disc drive apparatus, the magnetic disc rotation is broken usually by means of mechanical braking. However, for actuation of a mechanical brake device, it is necessary to reduce the rotation speed of the disc drive motor below a predetermined rotation speed. If the mechanical braking is done early, the brake-shoe must soon be replaced by a new one due to wear. Therefore, actuation of the mechanical brake device should be considered based on measuring the rotation speed both mechanically and electrically. However, it is not feasible in view of the large size of the total apparatus, the expense, and the inherent measuring error to measure rotation speed mechanically. On the other hand, upon braking it is impossible to measure speed electrically without providing another power source, because an electrical measurement of rotation speed must be done after a mechanical brake apparatus has cut off the power source to the total apparatus. Moreover, in the event that only regenerative braking is employed wherein motor coils are short circuited, there is a danger of damage to the disc and the magnetic head drive apparatus because at low speed regenerative braking is ineffective, resulting in contact of the magnetic disc with the magnetic head while the disc is still rotating and damage thereto.
Another possible method is to use reverse regenerative braking which is produced by providing the motor with reverse current in phase when braking is intended. One advantage of this method is that a larger torque is generated by the regenerative braking, and more power is available for braking even if the motor rotates slowly. However, there is a risk that the motor will begin to rotate reversely if the reverse current is not cut off before the motor has complete stopped. The disadvantages of this method are that it is too difficult to detect the instant the motor becomes completely stopped and it is necessary to provide the motor with the reverse current. Therefore it becomes ineffective if the power source is cut off.